4.2.1 Ameskar
Sap flow measurements on Aspen trees (Populus canescens) were conducted over multiple weeks in the Ameskar and Tichki Oases in the High Atlas Mountains (chapter 2.1). Selected daily results are presented in this section. Figure 29 shows diurnal measurements on Aspen trees at Ameskar from April 5th to 9th, 2005, with a ten-minute measurement interval. The measurements were taken automatically and the results were calculated by the integrated program PROSALog (UP-GmbH 2000). Channels one to three in the following correspond to the experimental data from aspen trees that were numbered one to three. The averaged leaf size (cm2) for all trees was estimated manually and was then input to PROSALog (Table 1, Table 2). The results of an identical field study on Aspen trees (Populus canescens) at Tichki are given in section 4.2.2. Note that channel (tree) three is not represented due to a permanent failure of the experimental equipment.
Three riparian Aspen trees were chosen within a radius of about five metres. In the Ameskar experiment, the trees were not as close to the water channel (Assif Ait Achmed) as in the Tichki experiment (next section). The distance to the perennial stream was about 50 m. Due to a bug in the system functioning that was not noticed until afterward, diurnal measurements in each channel stopped at 6 pm and reset at midnight the next day. This explains the apparently sharp decline in sap flow seen in the figure. The sap flow values (l m-2 leaf area) of channels (trees) one and two showed similar changes over the period of measurement.
A sharp increase in sap flow was observed during the morning. Sap flow declined around midday and increased again during the afternoon. Channel two showed higher sap flow relative to channel one. Initially, channel two reported sap flow intensities at peak of approximately 100 l m-2, whereas channel one reported about 50 l m-2 during the same time period. The differences between the channels were substantially smaller on the second day.
This was mainly due to a decrease in channel two to a maximum sap flow of 75 l m-2, while channel one maintained a maximum of 50 l m-2. The period from April 7th to 8th, 2005 was characterised by a more erratic measurement due to the system error mentioned above. In general, measurements were slightly lower for both channels compared to the previous days.
Finally, on April 9th, sap flow increased for both channels compared to the previous day.
Values for channel one increased to about 50 l m-2, while values for channel two increased to 90 l m-2, potentially due to greater growth.
0 20 40 60 80 100 120
Date-hours
l m-2 per leaf area
Channel 1 Channel 2
24:00h 06.04.
24:00h 07.04.
0:00h 24:00h 05.04.
24:00h
09.04.
08.04.
Figure 29: Sap flow measurements (l m-2 leaf area) on Aspen at Ameskar from April 5th-9th, 2005.
Measurements were taken every ten minutes. Channel one and two correspond to individual trees: Aspen one and Aspen two.
Climate parameters (source: IMPETUS Imeskar station) including maximum radiation
(W m -2), average relative humidity and average temperature are shown in Figure 30. The average relative humidity and the low temperature values both show moderate change over time. It should be noted that this climate station is located at 2300 m a.s.l., about 200 m higher than the experimental site. Thus, the lower temperature and relative humidity and the higher irradiance might be expected. These figures are provided to facilitate interpretation of the sap flow results, but are of limited application due to the altitudinal difference. Greater irradiance was observed on April 8th, 2005 than on the days before and after. Sap flow measurements depend mainly on trends in light availability, so sap flow values are expected to parallel highs and lows of irradiance. However, irradiance levels in the oases are lower than those shown in Figure 30 and are more similar to the measurements from Tichki (Figure 32). However, relative humidity values are lower at Tichki station, so those data are also less useful due to the altitudinal difference. Precipitation did not occur during the time span of the experiment.
0 200 400 600 800 1000 1200 1400
5.4.05 6.4.05 7.4.05 8.4.05 9.4.05
Date radiation (W m-2 )
0 5 10 15 20 25 30
rel. humidity (%), Temperature (°C)
maximum radiation relative humidity avg. temperature
12:52 PM 15:42 PM
18:25 PM
10:06 AM
16:25 PM
Figure 30: Automatic climate station measurements at Imeskar from April 5th-9th, 2005; daily averages of maximum radiation (W m-2) with the corresponding times of day, relative humidity (%) and average temperature (°C) are shown.
4.2.2 Tichki
Figure 31 shows diurnal measurements on Aspen trees at Tichki from June 21st to 25th, 2005, with a 10-minute measurment intervall. The entire experiment lasted from June 18th to August 25th, 2005. The peaks seen in all three channels at noon on June 24th and 25th were caused by a measurement error from 10 am to 2 pm. All three channels were blocked because the recording apparatus had failed.
Important features here are the distinct midday depressions from 12 pm to 3 pm following the noon peak for every experimental aspen. The consistent changes in amplitudes of all channels are also remarkable. Channel one showed a minor daily fluctuation of 45 l m-2 amplitude, compared to channel two and three with fluctuations of about 110 l m-2 and about 300.0 l m-2, respectively.
Channel three consistently showed higher sap flow than the other two. This may be due to the position of Aspen three at about one metre from the perennial stream; the other trees were located five metres and ten metres from the stream, respectively. Another possible cause is flooding that produced a permanent oversupply of water. All three trees reduced sap flow to low levels during the night, but a continuous increase occurred after daybreak. Values
for channel two steadily increased up to June 25th,2005. From June 21st to 24th, channel two reached maximum sap flow values of about 117 l m-2. Sap flow then increased rapidly to a maximum of approximately 272 l m-2 on the afternoon of June 25th. Initially, maximum sap flow rate of channel three was 293 l m-2 on June 21st. Subsequently, sap flow increased to a peak of 325 l m-2 at noon on June 23rd. Initial amplitudes showed a clear midday depression with a subsequent afternoon increase. The midday depression can be ignored due to the errors mentioned above.
0 50 100 150 200 250 300 350
date-hour l m-2 per leaf area
Channel 1 Channel 2 Channel 3
0:00h 24:00h 21.06.
24:00h 25.06.
24:00h 22.06.
24:00h 23.06.
24.06.
Figure 31 Sap flow measurements (l m-2 leaf area) on Aspen at Tichki from June 21st-25th, 2005.
Measurements were taken every ten minutes. Channels one, two and three correspond to individual trees: Aspen one, Aspen two and Aspen three.
These data can be compared to solar radiation (W m-2) and relative humidity (%) measurements from the automatic climatic station (Campbell Ltd.) (Figure 32) at Tichki village. This climate station is the nearest available, located about 100 m from the sap flow experimental site.
The relatively high night-time humidity between June 21st and 23rd, 2005 suggests a high precipitation potential. The climate station recorded 15 mm and 48 mm of precipitation for June 21st and 22nd, respectively. The differences between night-time and day-time sap flow measurements indicate the individual water status of the trees. Hence, we assume that the water status of each aspen tree measured is relatively high. The observed meteorological data for June 24th and 25th show consistently high irradiance with fewer declines in irradiance and lower humidity levels. Hence, the increased sap flow on those days may reflect the high irradiance. The relatively low humidity also promotes leaf-atmosphere exchange of water
vapour. Thus, climatic conditions induce aspen trees to increase their sap flow in order to adapt their water consumption to changing atmospheric vapour pressure.
0 10 20 30 40 50 60 70 80 90 100
days relative humidity (%) Temperature (°C)
0 500 1000 1500 2000 2500
solar radiation (W m-2 ) relative humidity Temperature solar radiation
19.06.2005 20.06.2005 21.06.2005
22.06.20
05 23.06.2005 24.06.2005 25.06.2005
Figure 32: Automatic climate station measurements of relative humidity (%), solar radiation (W m-2) and temperature (°C) from June 21st-25 th, 2005 at Tichki Oasis.
4.2.3 Comparison of the sap flow experiments at Tichki and Ameskar
Diurnal patterns observed in the Ameskar experiment were comparable to those observed in the Tichki experiment, except for the range of sap flow values, which were about twofold higher in Tichki than in Ameskar. One reason for this may be the relative proximity of the experimental trees to the perennial stream. The experimental trees at Tichki were located at about one to ten metres from the perennial stream. The slope of this stream is relatively high, and flooding after heavy rainfall at higher altitudes is common. This may explain the higher sap flow measurements. A second explanation is the seasonal difference between the two experiments. The Ameskar experiment was conducted during the spring, with relatively low temperatures and higher rainfall compared to summer conditions. The Ameskar oasis is located in a narrow part of Assif valley and thus shaded, which may influence sap flow.
Alternatively, the Ameskar experiment may have been influenced by surrounding Aspen trees, as it was conducted in a small forest. At Tichki, in contrast, the experiment was conducted on an open hillside with little surrounding vegetation. The Tichki oasis is also
located in a broader valley structure at the junction of multiple valleys (Figure 1). However, heavy rainfall at higher altitudes is not unusual during the summer and may contribute to increased water consumption by trees. Moreover, the Tichki experiment took place in June with relatively high irradiance on cloud-free days, together with high relative humidity and temperatures. The relatively high humidity, together with the high water vapour deficit, creates a greater demand for water and thus increase sap flow. Both experiments showed low evaporative demand at night and high water consumption during the day.
4.2.4 Comparison of simulated versus measured stomatal conductance rates
Figure 33 shows a comparison between simulated values of actual stomatal conductance (Cs-act) per km2 for the “deciduous tree” plant group type in the High Atlas Mountains and the results of the Tichki and Ameskar Aspen experiments. Simulated values are based on climate station data. Simulated Cs-act shows a strict annual cycle with a minimum in summer and a maximum in winter. Due to the experimental design, which covered only a few months, we cannot show sap flow measurements for the full year. For April and June 2005, simulated Cs-act overestimates the measured amounts. For August, the measured values are underestimated. Only few measurements are available for March, so no reliable comparison can be made. One potential error is due to the aggregation of the measured data to a monthly scale. The upscaling of diurnal stomatal conductance values of single trees to monthly figures is critical, because individual values largely depend upon site-specific recent meteorological conditions. Moreover, due to the spatial heterogeneity of trees, planted as narrow bands in mountain valleys (oases), comparisons to point data are difficult. A simulated 1-km2 cell located at the geographic position 31.315°N and -6.429°E may not correctly match the geographic position of the field experiment. Therefore, the stomatal conductance values are only a rough fingerprint of mountain plant's water usage.
The simulated actual stomatal conductance (Cs-act) monthly values are probably overestimated due to overestimated tree biomass amounts (see section 2.5.1.4) during model parameterisation. Thus, the model simulates stomatal conductance values incorrectly on an annual basis for the mountainous region. Exceptional levels of stomatal conductance (e.g., for April 2005) are not reproduced by any model. These values result from the high amplitude of measured sap flow in April. However, a more important feature of the model is its reflection of the annual frequency of high stomatal activities in winter compared to summer. The model plausibly simulates these figures.
In the High Atlas Mountains, however, water is not the limiting resource; streams are continuously supplied by wells, snow and rainfall (Schulz 2007).
0 50 100 150 200 250 300 350 400 450 500
Jan Feb Mar Apr Mai Ju
n Ju
l Aug Sep Oct Nov Dec
2005 mmol m-2 s-1
Simulated Stomatal conductance Measured sap flow
Figure 33: Simulated monthly actual stomatal conductance Cs-act (mmol m-2 s-1) of the
“deciduous tree” PFT group in the High Atlas Mountains compared to aggregated measurements from aspen trees at Tichki and Ameskar in 2005.